This invention relates to a method for reducing the amount of microfoam in a spray-applied waterborne polymeric composition. And, a method for increasing the density of a spray-dried waterborne polymeric composition is provided.
Waterborne polymeric compositions such as clear or pigmented coatings are frequently applied to substrates by spraying techniques. Some of the common spraying techniques used are, for example, air spray, airless spray, air-assisted airless spray, disc and bell spray, and high volume low pressure spray. In air spray compressed air is critical to atomize the waterborne polymeric composition and to assist in conveying the droplets to the substrate. In airless, disc, and bell spray techniques the waterborne polymeric composition is atomized by mechanical means and the droplets are suspended in air on formation. Air-assisted airless spray is a hybrid of the two spray methods described above wherein air is used to convey the droplets which are formed mechanically; and high volume low pressure spray is another commonly recognized variant of air spray. In each case air is involved in the atomizing and/or conveying aspect of the spraying methods.
Waterborne compositions containing water-soluble or water-insoluble polymers frequently contain microfoam when they are applied to substrates by various spraying techniques using air in the spraying process. Microfoam remaining in a dried film formed from the spray-applied aqueous composition may detract from the appearance of the film, particularly from the appearance of a clear, or unpigmented, film.
And, in another embodiment waterborne compositions containing water-soluble or water-insoluble polymers may be spray dried to form dry powders. In certain uses of such dry powders it is advantageous that the powder have as high a density as possible.
U.S. Pat. No. 4,842,900 discloses a method and apparatus for coating a liquid coating composition to a moving web in which air entrained on the web is replaced by a gas having a higher solubility than air in the liquid coating composition such as, for example, carbon dioxide.
U.S. Pat. No. 3,661,605 discloses an aqueous aerosol composition composed of a homogenized dispersion of lecithin in water and contains a propellant, eg., carbon dioxide or nitrous oxide, for pressurizing the composition in an amount sufficient to produce a release coating of lecithin on a surface when the composition is sprayed. Further disclosed is spraying an aqueous dispersion of lecithin by other means such as squeezing from a flexible bottle.
U.S. Pat. No. 4,869,878 discloses a device for the creation of an oxygen-free working atmosphere such as a device for the production of propellant and buffer gasses such as combustion waste gas for a spray gun with which molten metal or ceramic is applied to a workpiece.
U.S. Pat. No. 4,923,720 discloses a liquid coatings application process and apparatus in which supercritical fluids, such as supercritical carbon dioxide fluid, are used to reduce to application consistency viscous coatings compositions to allow for their application as liquid sprays.
Japanese Patent Application No. 60/85929 discloses a spraying composition containing a water-soluble resin, including vinyl acetate resin, and an Oxygen-blocking agent such as silicone oil. The propellant gas used in spraying is fluorocarbon or carbon dioxide.
United Kingdom Patent Application No. 2,020,308 discloses a propellant for aerosol spray cans, particularly lacquer spray cans, which is based on the use of carbon dioxide/ propane/ butane mixture.
None of the references discloses a method for reducing the amount of microfoam in a spray-applied waterborne composition or increasing the density of a spray-dried waterborne composition.
A method for reducing the amount of microfoam in a spray-applied waterborne polymeric composition is provided, wherein a gas having a solubility factor greater than about one is used in spraying the waterborne composition. And, a method for increasing the density of a spray-dried waterborne polymeric composition is provided, wherein a gas having a solubility factor greater than about one is used in spraying the waterborne composition.
This invention is directed to a method for reducing the amount of microfoam in a spray-applied waterborne polymeric composition by spraying the waterborne composition using a gas having a solubility factor greater than about one.
A xe2x80x9cwaterborne polymeric compositionxe2x80x9d herein is defined as a composition containing a soluble polymer dissolved in or an insoluble polymer dispersed in a medium which is predominantly composed of water, or mixtures thereof. The waterborne polymeric composition may contain a solution of a polymer in water or in an aqueous medium, which aqueous medium may be, for example, a mixture of water and at least one water-miscible solvent such as, for example, isopropanol, ethylene glycol butyl ether, and propylene glycol propyl ether; the waterborne polymeric composition may contain an alkali-soluble polymer dissolved in an alkaline aqueous medium; or the waterborne polymeric composition may contain a dispersion of an insoluble polymer in an aqueous medium such as, for example, an emulsion-polymerized polymer.
The polymer in the waterborne polymeric composition may be prepared by the addition polymerization of at least one ethylenically unsaturated monomer such as, for example, esters of (meth)acrylic acid, vinyl esters, styrene, and butadiene. Polar monomers such as (meth)acrylic acid, itaconic acid, acrylonitrile, dimethylaminoethyl (meth)acrylate, and hydroxyethyl (meth)acrylate may also be incorporated in the polymer. Low levels of multi-ethylenically unsaturated monomers may be incorporated in the polymer. Chain transfer agents such as, for example, mercaptans may be used in order to moderate the molecular weight of the polymer. When the polymer is in the form of an emulsion-polymerized polymer, the particles may be from about 40 nanometers to about 4000 nanometers in diameter.
The solids content of the waterborne polymeric composition may be from about 20% to about 70% by weight. The viscosity of the waterborne polymeric composition may be from about 50 centipoises to about 10,000 centipoises, as measured using a Brookfield viscometer (Model LVT using spindle #3 at 12 rpm); the viscosities appropriate for different spraying methods vary considerably.
The waterborne polymeric composition may contain, in addition to the polymer, conventional components such as, for example, emulsifiers, pigments, fillers, dispersants, coalescing agents, anti-migration aids, curing agents, thickeners, humectants, wetting agents, biocides, plasticizers, antifoaming agents, colorants, waxes, and anti-oxidants.
A spray-applied waterborne polymeric composition is a composition which has been applied to a substrate such as, for example, metal, wood, and plastic, using a spraying method. Preferred is a composition which has been applied to a wood substrate such as, for example, wood, sealed wood, particle board treated with a UV-cured filler, painted wood, and previously coated wood; or to an automotive substrate such as, for example, plastic, reinforced plastic (such as RIM substrate), metal, treated metal, metal coated with an electodeposited primer, and previously painted metal; by a spraying method such as, for example, air-assisted spray, airless spray, bell or disc spraying, high volume low pressure spray, and air-assisted electrostatic spray. In spraying methods the waterborne polymeric composition is atomized, or formed into small droplets, which are conveyed to the substrate where the droplets form into a substantially continuous structure such as, for example, a film. In such spray-applied methods the atomized droplets of the waterborne polymeric composition are formed in contact with and/or admixed with a gas-conventionally, air. The gas, under pressure, may be required to atomize the coating such as, for example, in conventional air spray applications; the gas may flow towards the substrate and provide for at least some of the conveying of the atomized composition such as, for example, in air-assisted airless spray application; or the gas may be the medium through which the atomized composition, atomized by mechanical action in the presence of the gas such as, for example, in airless spray, disc, and bell applications, with or without electrostatic assistance, moves to the substrate. Occlusions of gas, xe2x80x9cmicrofoamxe2x80x9d, typically are found in the waterborne polymeric composition after its application to the substrate when using air as the gas in the process. The microfoam is undesirable; microfoam may cause haze or opacity in clear or substantially unpigmented coatings or films; and microfoam may lower the clarity, depth, gloss, or xe2x80x9cdistinctness of imagexe2x80x9d of clear or pigmented coatings or films. In the method of this invention microfoam which has formed within the applied waterborne polymeric composition is reduced in amount by using a gas having a solubility factor greater than about one in place of air. xe2x80x9cUsing a gas having a solubility factor greater than onexe2x80x9d as used herein means that at least one gas having a solubility factor, as defined herein, greater than about one is used in the spraying process in place of the air commonly used. The gas having a solubility factor greater than about one may be used under pressure where such is required by the spraying method selected; or the gas having a solubility factor greater than about one may be used at atmospheric pressure in the area where the atomized droplets are formed and admixed with gas. Preferred is using a gas with a solubility factor greater than about 35. Most preferred is using carbon dioxide gas.
xe2x80x9cMicrofoamxe2x80x9d herein is defined as substantially spherical gas-filled occlusions which are typically 10-20 micrometers in radius. The microfoam occlusions lack sufficient buoyancy to escape from the waterborne polymeric composition before they become substantially immobilized in the applied composition. The xe2x80x9camount of microfoamxe2x80x9d as used herein is determined by counting the number of bubbles in a selected area of the applied waterborne composition, using an optical microscope under 70X magnification.
The method of this invention includes spraying the waterborne polymeric composition using a gas having a solubility factor greater than about one. The xe2x80x9csolubility factorxe2x80x9d of a gas as used herein is defined as:       solubility factor of a gas    =            solubility of the gas in water                  solubility of nitrogen in water      
wherein solubility is measured at 25 C. at 1 atmosphere gas pressure.
Some solubility factors are presented below:
And, in another embodiment, a method for increasing the density of a spray-dried waterborne polymeric composition is provided, wherein a gas having a solubility factor greater than about one is used in spray-drying the waterborne composition. Carbon dioxide is the preferred gas. In some uses a spray-dry prepared powder of increased density is desired. For example, in the ceramic industry the operation of dry pressing requires dry granules of ceramic powder mixed with dispersants and binders. Denser powder granules have improved flow properties and fill dry press molds better than less dense powders. Denser powder granules are formed when aqueous slips containing pigment, dispersant, and polymeric binder are spray-dried using carbon dioxide rather than nitrogen as the atomizing and carrier gas.